The present invention relates generally to the protection of integrated circuits (ICs) from electrostatic discharge (ESD), and more particularly to an improved ESD structure for protection against high voltage ESD events.
As IC devices continue to shrink, they become more susceptible to ESD damage. ESD events occur when charges are transferred between one or more pins of an IC device and a conducting object in a short period of time, typically less than one microsecond. The rapid charge transfer often generates voltages large enough to destroy such insulating films as silicon dioxide layers, and also causes permanent damage to IC devices. To cope with ESD-related problems, IC manufacturers have designed various ESD structures to protect IC devices from ESD damage.
In a typical IC device, metal-oxide-semiconductor (MOS) devices without effective ESD protection can be easily damaged by ESD current because their thin gate oxide layers can be easily destroyed by voltages generated during an ESD event. Electrostatic voltages from common environmental sources can reach thousands or even tens of thousands of volts. Such voltages can be destructive even if their resulting current level is extremely small. For this reason, it is critical to discharge any electrostatic charge before it accumulates to a damaging voltage.
Although conventional ESD structures for protecting a MOS device against ESD damage are commonly available, they may not efficiently dissipate high voltage (HV) ESD current because the resistance across the substrate of the MOS device is high. For example, a silicon-controlled-rectifier (SCR), a conventional ESD structure, formed across the substrate cannot be triggered easily at a low voltage because of the high substrate resistance. This may cause heat to accumulate within the ESD structure, resulting in damage to the MOS device. Many attempts have been made in the past to improve HV ESD protection by simply increasing the dimension of the ESD structure. However, this approach turned out to be ineffective because it merely generates an ESD structure with a gradient doping profile that does not dissipate ESD current efficiently.
Therefore, there is a need for an ESD structure for dissipating ESD current more efficiently to improve ESD performance.